Methodology

In this analysis,  each parcel is examined individually, and then estimates are made of how many units can be built in that parcel.   I chose the parcel methodology because I believe that it gives a more accurate representation of how much residential growth could occur. A comparison of different methods of estimated buildout is available on this site.

The basic idea of a parcel buildout is to estimate how many units can fit on each parcel. There are many ordinances that might influence the number of units allowed on a parcel. This process had to be simplified in order to apply it to all 6,000 parcels in Charlestown. These simplifications are specifically discussed in the assumptions section.

The basis for the buildout analysis is the density equation in Charlestown's zoning laws. Article XI 218-59 C states that the "The maximum number of developable lots in a residential cluster subdivision shall be computed using the following formula:"

(Total Area - Constraints to Development) / Minimum Lot Size = Maximum Number of Lots

This formula can be applied to every parcel in town by utilizing the ArcInfo GIS program. The ArcInfo Macro Language program that is used to do this calculation is available on this site. The following is a list of steps taken to determine the density of residential development that could occur in each parcel:

One parcel is selected individually and the area within that parcel is determined. This example shows the selection of a parcel in the North West corner of Charlestown. To see the selection, click on the map.
The zoning of the parcel is then determined. If the parcel is in more than one zone, each zone is treated separately. In this example, the area in the parcel that is zoned 3 acres is selected. A list of the zoning in Charlestown is available. Click on the map to see the area that is zoned Residential 3 Acres.
Next, the area of the constrained land is calculated. This is accomplished by selecting the land that is part of the constrained coverage that also is part of the parcel in the particular zone that is under consideration.. Click on the map to see the are of constrained land within the area that is zoned R-3A in the parcel.
In Charlestown's zoning regulations, constrained land is not included in the total area for determining the number of units that can go in a particular parcel. The next step is to subtract constrained land from the total land in the parcel that lies in the specific zone. The result is the area of land that can be built on. This land is considered the "goodland" or the "buildable land". The total area of the buildable land is then calculated from this selection. Click on the map to see the the buildable land in the R-3A section of this parcel.
The area of buildable land is then divided by the zoning area to determine a preliminary number of allowed units.  This is used to determine if the land is could be subdivided.  All parcels that can be subdivided are assumed to be subdivided. This provides an estimate of the maximum possible number of units that can be built on a parcel. If the preliminary number of units that can be built on the parcel is greater than 2, 15% of the area that is goodland is taken out of the total buildable land for roads that would be necessary for a subdivision. The number of units that can be built on the parcel is then recalculated with area taken out for roads

The predicted number of units on a particular parcel is then compared with the existing tax assessment data. If this method determines that there no units can be built on a parcel, and the tax records indicate that there is an existing unit on the property**, the existing unit will remain in the buildout scenario. Otherwise, the number of units predicted is used as the number of units at buildout. This calculation is repeated for all of the potential residential zones that could be in the parcel, and the sum is taken for all of the zones. This value is then set the as the total number of units for the parcel.

This methodology  is repeated for every parcel in Charlestown. The sum of the number of units that is computed for each parcel is the estimation of the total number of units that can be built in Charlestown under current land use policies i.e. the "buildout".  Please read the assumptions section so that you can better understand the uncertainty involved in this methodology.

* A unit is one dwelling. It can either be a house or apartment.

** The Tax Codes only codes parcels from having one unit, or more than one unit. A copy of these codes is available. Assumptions were made for tax codes that allow a range in the number of units that exist in the parcel. A table listing these assumptions is available.

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